Study On Surface-interface Structure Regulation And Photoelectrochemical Water Splitting Properties Of InGaN Nanorods

Hydrogen production via solar photoelectrochemical（PEC）water splitting technology is one of the most promising methods to solve the energy crisis.As the key component of the PEC cell,the photoelectrode semiconductor material limits the solar to hydrogen conversion（STH）efficiency.InGaN nanorods with high carrier mobility,tunable band gap（3.4 e V～0.69 e V）,suitable band edge potential,and large reactive area are ideal photoelectrode for high-efficiency hydrogen production,showing great application prospects for PEC water splitting.However,some problems leading to the serious charge recombination of InGaN nanorods and limiting the PEC performance still remain:i)The difficults of controlling the In composition and morphology of InGaN nanorods result in a few defects exsit during the growth process;ii)The large lattice mismatch between the InGaN nanorods and the substrate leads to a poor interface,which makes carrier transport difficult;iii)The large specific surface area of the InGaN nanorods introduces surface states,which serve as the center of carriers recombination and reduces the carrier transfer efficiency.Hence,this thesis reported the doped method,novel substrate and heterojunction construction of InGaN nanorods from the perspective of surface-interface structure regulation of InGaN nanorods to improve the crystal quality of InGaN nanorods and eliminate the interface resistance between the nanorods,the growth substrate and the electrolyte,thereby promoting the carrier separation efficiency and comprehensively enhancing the PEC performance of the InGaN nanorods.The detailed research work and results are as follows.First,from the perspective of surface structure regulation of InGaN nanorods,the MBE self-assembly method combined with in-situ Zn doping technology has been applied to prepare InGaN nanorods with tunable In composition and the electronic structure,significantly improving the PEC performance of InGaN nanorods.The growth kinetics study of Zn-doped InGaN nanorods indicates that the introduction of Zn atoms can significantly reduce the adsorption energy of In atoms and thus reduce the incorporation of In,which is beneficial to the substitutional doping of Zn atoms.Meanwhile,the dislocation defects,segregation of high In composition and the merger of of Zn-doped InGaN nanorods have been supressed,which ffectively eliminats the Fermi level pinning effect on the surface of nanorods,inhibits the bulk and surface carrier recombination,and thus promotes the charge separation and transfer between the photoanode/electrolyte interface.On the other hand,Zn doping increases the bandgap of the InGaN nanorods and improves the corresponding valence band potential,which results in favorable band bending and enhances the oxygen evolution reaction（OER）kinetics of the nanorods.Therefore,the improvement of the crystal quality and the change of the electronic structure synergistically improve the photovoltage of the InGaN nanorods and enhance the driving force of OER.As a result,the optimum Zn-doped InGaN nanorods photoanode showed the photocurrent density of 0.8 m A/cm² at 1.0 V vs.RHE（Reverse hydrogen electrode）,which is 3 times than InGaN nanorods（0.24 m A/cm²）.In addition,compared with pure nanorods,the onset potential of Zn-doped InGaN nanorods is0.7 V vs.RHE,showing a negative shift of 280 m V.After further appling Au nanoparticles cocatalyst modification,the prepared Zn-doped InGaN nanorods@Au photoanode has a photocurrent density of 11.5 m A/cm² at 1.23 V vs.RHE and a maximum ABPE（Applied bias photoconversion efficiency）of 1.33%.Second,from the perspective of interface structure regulation between InGaN nanorods and the growth substrate,we explored a high-conductivity 2D MXene film on the Si substrate as the growth substrate by a simple chemical lift-off method to prepare InGaN nanorods with uniform morphology and high quality,and thus obtain InGaN/MXene/Si heterojunction photoanode with efficient carrier separation.Experimental and theoretical studies have shown that a Schottky junction and an ohmic junction among InGaN/MXene and MXene/Si interface are formed,making MXene an ideal electron transmission channel.MXene promotes the photogenerated electrons transfer from InGaN nanorods to Si substrates and then mmigrate to the counter electrode,thereby boosting the charge separation and transfer process.The synergistic effect of MXene significantly reduces the interfacial resistance of InGaN/Si and InGaN/electrolyte and improves the rapid hole injection efficiency（82%）of the photoanode surface,thus avoiding the holes accumulation on the electrode surface and improving the stability against light corrosion of InGaN nanorods.As a result,the optimum InGaN/MXene photoanode shows the photocurrent density of 7.27 m A/cm²at 1.23 V vs.RHE,and the maximum ABPE of 2.36%at 0.63 V vs.RHE,which is～10 times higher than InGaN nanorods directly grown on Si substrate.Furthermore,the InGaN/MXene photoanode has an ultra-low onset potential of 75 m V vs.RHE,which is lower than the current InGaN or Si-based photoanodes that have been modified by the interface modification.This method broadens a new direction for the growth of III-nitride nanorods,and provides novel insight for the design of high-efficiency optoelectronic devices by integrating multi-scale materials.Finally,from the perspective of interface structure regulation between InGaN nanorods and the electrolyte,the Cu₂O semiconductor with excellent conductivity was used to couple the InGaN nanorods to construct a Z-type heterojunction with enhanced carrier transport,achieving high-efficinecy PEC water splitting of InGaN nanorods under zero-bias voltage.The introduced Cu₂O has opposite effects on the optical and PEC performance of the InGaN/Cu₂O heterojunction.The resulting optimal InGaN/Cu₂O photoelectrode exhibits the highest photocurrent density（6.3 m A/cm²）at 1.23 V vs.RHE,which is～12.5 times higher than the pure InGaN nanorods photoelectrode（0.5 m A/cm²）.However,the self-oxidation of InGaN/Cu₂O under external bias leads to poor stability of the heterojunction photoelectrode.The carrier dynamics simulation calculations of InGaN/Cu₂O heterojunction show that the electrons of InGaN nanorods and holes of Cu₂O are depleted,and the remianing holes and electrons are accumulatd at the heterogeneous interface.XPS analysis indicates that the electrons in the heterojunction are transferred from InGaN nanorods to Cu₂O.Combined with the band structure of the heterojunction,a typical Z-type InGaN/Cu₂O heterojunction is revealed.The recombination of the electrons of InGaN nanorods and holes of Cu₂O effectively promotes electron transfer,thereby eliminating the accumulation of holes on InGaN/Cu₂O,which not only improves the ability of electrons to reduce water to hydrogen,but also enhances the stability of the electrode.As a result,InGaN/Cu₂O shows the photocurrent density of～170μA/cm²,which is 8.5 times higher than InGaN nanorods without an external bias.Meanwhile,the InGaN/Cu₂O photoelectrode has excellent stability against photocorrosion.In conclusion,the regulated strategy of the surface-interface structure of the InGaN nannrods can effectively reduce the carrier recombination of the nannrods,promote charge separation and transfer,and enhance their PEC performance.This thesis provides certain guiding significance for the discovery of surface interface structure regulation strategies for new semiconductor material,and promotion of the development and application of hydrogen production via photoelectrochemical water splitting.